Justification:Mammillaria marcosii is considered to be Critically Endangered due to its very small range, its occurrence in a single location, a small population size, and rapid and ongoing decline in population size estimated to be greater than 80% over the next 3 generations (generation length is estimated at 5–10 years). Furthermore, it does not occur in any protected area. The population consists of fewer than 150 plants, which are found in a single small area of less than 10 km². Since this species was discovered in 1996 (Fitz Maurice et al. 1997), the population has diminished by an estimated 90% due to illegal collecting.

The current population is estimated to be less than 150 individuals. Since this species was discovered in 1996 (Fitz Maurice et al. 1997), the population has diminished by an estimated 90% due to illegal collecting.

This species is listed on CITES Appendix II, but enforcement of the laws in the countries of destination is needed. Efforts to protect this species from illegal collecting are urgently needed to stop the rapid decline of the wild population. It does not occur in any protected area.